I keep having this feeling I'm being followed... Must be nerves from being with you for so long
Although it probably wasn't. Surely John would trust his instincts (which were usually correct) and know that there really was someone following him.
Sherlock had been afraid that this would happen. He should never had met with John. It was a huge mistake and someone would have to pay for it. Hopefully, it wouldn't be John.
Sherlock searched his mind palace for an answer as to whom it was that would keep an eye on John and probably kill him if Sherlock came in contact with him again.
It took him several days, but he finally came up with a conclusion. The man in their old flat. He looked strangely familiar, and there was also a man who seemed to be near John when Sherlock found him that looked uncannily like the man in the flat.
Sherlock snatched up his laptop.
The rest of the night would be used researching this strange character.